Motor vehicles are increasingly being fitted with driver assistance systems, for example with so-called ACC (adaptive cruise control) systems, which allow the distance of the driver's own vehicle from a vehicle in front to be automatically regulated. To that end, the distances and azimuth angles and also the relative speeds of vehicles travelling in front are measured with the aid of the radar sensor, for example an FMCW-LRR (frequency modulated wave long range radar) sensor.
The radar sensors used hitherto in that connection have, for example, four antenna elements or antenna patches which are arranged horizontally offset from the optical axis of the radar sensor in front of a radar lens.
In a first evaluation stage, each antenna element is allocated exactly one channel in which the signal received by the relevant antenna element is evaluated. For example, in the case of an FMCW radar in which the frequency of the transmitted radar signal is modulated periodically, for each antenna element the received signal is mixed with the signal transmitted at the time of reception, so that an intermediate frequency signal is obtained whose frequency corresponds to the frequency difference between the transmitted signal and the received signal. In the relevant channel of the evaluation device, a frequency spectrum of the intermediate frequency signal is then recorded in each measuring cycle. In that frequency spectrum, each located object shows as a peak whose frequency is dependent upon the distance and relative speed of the object concerned. By modulation of the transmitted frequency using different ramp slopes, it is possible for the distance and the relative speed to be calculated from the frequencies obtained.
Each antenna element radiates the radar output into a certain solid angle region at an intensity that varies as a function of the azimuth angle. The amplitude and phase of the signal received is dependent upon the azimuth angle of the located object. That dependency, the so-called angle characteristic, may be represented for a standard object at a given distance and with a given reflective power in an antenna diagram. By reconciling the amplitudes and/or phases obtained by the various antenna elements for the same object with the corresponding antenna diagrams, it is then possible to determine the azimuth angle of the object concerned.
The antenna elements are mounted behind a radar lens or some other covering, a so-called radome, and are therefore protected against effects of the weather. A film of water or a coating of dirt on the lens or radome may, however, cause the radar radiation to be attenuated to such a great extent that the object peaks no longer have a sufficient signal-to-noise ratio and therefore reliable object detection is no longer possible. This is referred to as “blinding” of the radar sensor. Since the driver of the vehicle normally relies on the functional capability of the driver assistance system, it is important for safety reasons that any impairment of operation or blinding of the radar sensor during operation may be detected without delay.
German Patent Application 199 45 268 A1 describes a method for detecting blindness, in which there is used as a blindness indicator inter alia the average angle quality of all detected objects. The term “angle quality” refers in that case to the quotient of the real object angle and the difference between the real and the measured object angle. If the angle quality averaged over a plurality of objects points to a poor quality of the angle detection, that is taken as an indication of possible soiling or blinding of the radar sensor.
With that conventional method, it is not possible, however, to detect a layer of ice on the radar lens or radome, since a layer of ice generally does not lead to significant attenuation of the radar signal.
However, since such a layer of ice has a refractive index that is different from the refractive index of air, it may lead, similarly to the radar lens itself, to a refraction and hence to a change in the direction of the radar radiation, with the result that, although objects may still be detected, the determination of their azimuth angle is falsified. That situation will be referred to hereinafter as “angle blindness”.